Various solutions and multiple optical combinations have been devised in the past for achieving coupling between a phototransducer and an optical fiber.
The document "Progress in monomode optical fiber interconnection devices", published in "Journal of Lightwave Technology", Vol. LT-2, No. 3, pp. 217-227, June 1984, analyzes various such solutions.
Overall, such known solutions can be classified into four categories.
As shown in FIG. 1, systems in the first category include a single discrete optical element 20 such as a lens that may be aspherical, spherical, or otherwise, that is interposed between a fiber 10 and a phototransducer 30. Such systems serve both to increase the numerical aperture and to increase the working distance dw between the phototransducer 30 and the coupled element 20. Nevertheless, such known systems are bulky because they use a discrete element 20 of large size. In addition, such systems are difficult to implement because they require two mounts and two separate X, Y, Z adjustments.
As shown in FIG. 2, systems of the second category have two discrete lenses 20 and 22 interposed between the fiber 10 and the phototransducer 30. This theoretically satisfactory solution makes it possible likewise to increase both numerical aperture and working distance dw. Nevertheless, when implemented, it is not entirely satisfactory. This configuration gives rise to a system that is very bulky: by using large-sized discrete optical elements 20 and 22 the length Lopt between the inlet face of the first lens and the fiber can easily reach 5 mm with a diameter that is equivalent. In addition, it is also difficult to implement this system. Three mounts and three separate X, Y, Z adjustments are required: one for the fiber 10 and two others for the lenses 20 and 22.
As shown in FIG. 3, systems in the third category comprise a microlens 20 placed on the end of the fiber 10 over its core 12. These systems are those that are in most widespread use. They generally require radii of curvature r of the order of 7 .mu.m for the microlenses 20 and working distances dw of the order of 10 .mu.m between the phototransducer 30 (e.g. a laser diode) and the fiber 10 fitted with its microlens. Nevertheless, these systems also suffer from various drawbacks. Microlenses 20 are difficult to make reproducibly (it is difficult to control the radii of curvature). This characteristic which degrades coupling is essentially due to the small radii of curvature required. The feedback on the emitting source 30 (laser diode) is large, thus disturbing its characteristics. This phenomenon is related to the small working distances dw imposed by this system. There are considerable risks for the phototransducer 30 due to the proximity of the microlens 30. In addition, the system can become dangerous for the phototransducer 30 if highly divergent beams are being used since the working distance dw then needs to be reduced even further in order to enable all of the light from the phototransducer to be collected. The accuracy required for alignment remains very high because of the small radii of curvature r and because of the small working distance dw.
As shown in FIG. 4, systems of the fourth category comprise a glass element 20 provided with a lens 22 that is interposed between the fiber 10 and the phototransducer 30. By optimizing the distance d1 between the fiber 10 and the outlet face of the glass element 20, and by optimizing the thickness d2 of the glass element 20, it is possible to double the radius of curvature r of the lens 22 and the working distance dw between the phototransducer 30 and the inlet face of the glass element 20 in comparison with microlens-fitted fiber systems as shown in FIG. 2. Nevertheless, these systems also suffer from certain drawbacks. Firstly, the increase in the radius of curvature r of the lens and in the working distance dw nevertheless remain relatively small. The configuration comprising a lens in association with a glass element thus continues to suffer, although to a lesser extent, to the same drawbacks as the system having a fiber fitted with a microlens, as shown in FIG. 3. In addition, if the distance d1 between the fiber 10 and the glass element 20 is chosen to be non-zero, then the system requires two mounts and two X, Y, Z adjustments, instead of only one.
Proposals have also been made in documents EP-A-0 155 379 and JP-A-31 89 607 for coupling devices based on pieces of graded index fiber. Nevertheless, such systems do not appear to have given rise to industrial applications.
In conclusion, known systems are not entirely satisfactory.